QUANT TECHNIQUES STRAIGHT FROM SERIAL CAT TOPPER BYJU

INDEX

1) POWER CYCLE

2) LAST 2 DIGITS TECHNIQUE

3) “MINIMUM OF ALL” REGIONS IN VENN DIAGRAMS

4) SIMILAR TO DIFFERENT GROUPING ( P&C)

5) APPLICATION OF FACTORIALS

6) GRAPHICAL DIVISION FOR GEOMETRY

7) ASSUMPTION METHOD

1)Power CycleThe last digit of a number of the form ab falls in a particular sequence or order depending on the unit digit of the number (a) and the power the number is raised to (b). The power cycle of a number thus depends on its’ unit digit. Consider the power cycle of 2 21=2, 25=32 22=4 26=64 23=8 27=128 24=16 28=256 As it can be observed, the unit digit gets repeated after every 4th power of 2. Hence, we can say that 2 has a power cycle of 2,4,8,6 with frequency 4. This means that, a number of the form 24k+1 will have the last digit as 2 24k+2 will have the last digit as 4 24k+3 will have the last digit as 8 24k+4 will have the last digit as 6 (where k=0, 1, 2, 3…) This is applicable not just for 2, but for all numbers ending in 2.

from 0 to 9, which are given below

Unit digit Power cycle Frequency

0 0 1

1 1 1

2 2,4,8,6 4

3 3,9,7,1 4

4 4,6 2

5 5 1

6 6 1

7 7,9,3,1 4

8 8,4,2,6 4

9 9,1 2

Therefore to find the last digit of a number raised to any power, we just need to know the power cycle of digits

For example 1) Find the remainder when 375 is divided by 5.1) Express the power in the form, 4k+x where x=1, 2, 3, 4. In this case 75 = 4k+3.2) Take the power cycle of 3 which is 3,9,7,1. Since the form is 4k+3, take the third digit in the cycle, which is 7Any number divided by 5, the remainder will be that of the unit digit divided by 5. Hence the remainder is 2.Sometimes, you may get a question in the term of variables, where you need to substitute values to get theanswer in the fastest way possible.For example,

2) Find the unit digit ofPut n=1, the problem reduces to 73^4, which is 781.

Since 81=4k+1, take the first digit in the power cycle of 7, which is 7.

3) What is the first non zero integer from the right in 83301957 + 83701982?a) 3 b) 1 c) 9 d) none of these

83701982 will end with more number of zeroes so we need to consider only the first part. Rightmost non-zero integer of the expression will be = unit digit of 8331957 = unit digit of 31957. Since 1957=4k+1, take the first digit in the power cycle of 3, which is 3.

4) If N = (13)1! + 2! + 3! + ..+ 13! + (28)1! + 2! + 3!..+ 28! + (32)1! + 2! + 3! + ...+ 32!+ (67) 1! + 2! + 3! + ......+ 67!, then the unit digit of N is

(a) 4 (b) 8 (c) 2 (d) none of theseBased on Power Cycle After 4! Every number is of the form 4k+4 , here we need to check the nature of the power till 4! Every term’s power is of the form 4k+1. So taking the first digit from the power cycles of 3,8,2, and 7 we will get the unit digit as (3+8+2+7 = ..0). Ans = 0

2) Useful technique to find the last 2 digits of any expression of the form ab

Depending on the last digit of the number in question, we can find the last two digits of that number. We can classify the technique to be applied into 4 categories

TYPE METHOD EXAMPLES

1) Numbers ending in 1 The last digit is always 1.

The 2nd last digit = product of tens digit of base * unit digit of the power. In 2167; 2 is the tens digit of base and 7 is the unit digit of power

1) 2167 =__41(2 * 7=__4_)

2) 4187 =__ 81( 4 *7= __8_)

3) 1261167 =__21 ( 6 * 7= __2_)

4) 31124 =__ 21( 3 * 4=__2_)

2) Numbers ending with 5 Last two digits: always 25 or 75

e.g.) 155534 = __ 25

3) Numbers ending in 3, 7, 9 Change the power so that the base ends with 1 and then use the same technique as for those numbers ending with 1.

eg) 34, 74&92 all will end in1.

e.g.) 17288 =(174)72 (taking the power 4as 74 will end in 1.(172* 172) 72

=( _89*_89)72(as last 2 digits of 172=89)

= ( _21)72 (as last 2 digits of 89*89=21) Answer =__41( as 2*2=4)

4) For even numbers (2,4,6,8) Use the pattern of the number 1024 =210 i.e. *210 raised to even power endswith 76 and* 210raised to odd power endswith 24.

e.g.) 2788 = (210) 78 * 28 =

__76 * __56 = __56.

It is also important to note that, 1. 76 multiplied by the last 2 digits of any power of 2 (>21)will end in the same last 2 digits as of that power of 2E.g. 76*04 = 04, 76*08 = 08, 76*16 = 16, 76*32 = 32

2. The last two digits of x2, (50-x)2 , (50+x)2 , (100-x)2 will always be the same. For example last 2 digits of122,382,622,882,1122…. will all be the same (..44). Also, last two digits of 112=392=612=892 =1112=1392=1612=1892 and so on

3.To find the squares of numbers from 30-70 we can use the following method

5) To find 412

Step1 : Difference from 25 will be first 2 digits = 16Step 2 : Square of the difference from 50 will be last 2 digits = 81 Answer = 1681.

6) To find 432

Step1 : Difference from 25 will be first 2 digits = 18Step 2 :Square of the difference from 50 will be last 2 digits = 49 Answer = 1849

4. Combining all these techniques we can find the last 2 digits for any number because every even number canbe written as 2* an odd number

3) “Minimum of all” regions in Venn Diagrams7) In a survey conducted among 100 men in a company, 100 men use brand A, 75 use brand B, 80 use

brand C, 90 use brand D & 60 use brand E of the same product. What is the minimum possible number

of men using all the 5 brands, if all the 100 men use at least one of these brands?

Sum of the difference from 100 = (100-100) + (100-75)+(100-80)+(100-90)+(100-60) = 95 Again take the difference from 100 = 5 (answer)

4) Similar to different Grouping in Permutation & CombinationAll questions in Permutation and Combination fall into 4 categories, and if you master these 4 categories, you

can understand all concepts in P&C easily.

1) Similar to Different

2) Different to Similar

3) Similar to Similar

4) Different to Different

So, for dividing 10 identical chocolates among 3 persons you can assume to have 12 (10 zeroes +2 ones)

things among which ten are identical and rest 2 are same and of one kind.So the number of ways in which you

In this booklet, we will look at the first category; i.e. Similar to Different, where I will give a unique approach to

the number of ways of dividing ‘n’ identical (similar) things into ‘r’ distinct (different) groups

a) NO LIMIT QUESTIONS

Let me explain this with an example. Suppose I have 10 identical chocolates to be divided among 3 people.The 10 chocolates need to be distributed into 3 parts where a part can have zero or more chocolates.

So let us represent chocolates by zeroes. The straight red lines (Call then “ones”) are used to divide them into parts. So you can see that for dividing into 3 parts, you need only two lines.

Suppose you want to give 1st person 1 chocolate, 2nd 3 chocolates and 3rd 6 chocolates. Then you can show it as:

Suppose you want to give one person 1 chocolate, another person 6 chocolates and another one 3, then it can

be represented as:

Now if first person gets 0, second gets 1 and third gets 9 chocolates then it can be represented as:

Now suppose you want to give first person 0, second also 0 and third all of 10 then you can show it like:

can distribute ten chocolates among 3 people is the same in which you can arrange 12 things, among which 10

are identical and of one kind while 2 are identical and of one kind which can be done in

The above situation is same as finding the number of positive integral solutions of a + b + c = 10. a, b, c is the

number of chocolates given to different persons.

b) LOWER LIMIT QUESTIONS

Now suppose we have a restriction that the groups cannot be empty i.e. in the above example all 3 persons

should get at least 1.

You have to divide ten chocolates among 3 persons so that each gets at least one. Start by giving them one each

initially and take care of this condition. You can do this in just 1 way as all the chocolates are identical. Now, you

are left with 7 chocolates and you have to divide them among 3 people in such that way that each gets 0 or

more. You can do this easily as explained above using the zeroes and ones. Number of ways =

The above situation is same as finding the number of natural number solutions of a + b + c = 7. (a, b, c are the

number of chocolates given to different persons)

Now suppose I change the question and say that you have to divide 10 chocolates among 3 persons in such away that the first person gets at least 1, the second at least 2 and the third at least 3.It’s as simple as the last one. First fullfill the required condition.Give the 1st person “1”, second person “2” and the third person“3” chocolates and then divide the remaining 4(10–1–2-3) chocolates among those 3This is same as arranging 4 zeroes and 2 ones which can be done in 6C2 ways.

The above situation is same as finding the number of positive integral solutions of a + b + c = 10 such that

a > 1, b > 2, c > 3. a, b, c is the number of chocolates given to different persons.In this case the answer is 6C2.

8) Rajesh went to the market to buy 18 fruits in all. If there were mangoes, bananas, apples and

oranges for sale then in how many ways can Rajesh buy at least one fruit of each kind?a) 17C3 b) 18C4 c) 21C3 d) 21C4

This is a Grouping type 1 Similar to Different question, with a lower limit condition. M+B+A+O=18 Remove one from each group, therefore 4 is subtracted from both sides. The problem changes toM+B+A+O=14. Using the logical shortcut you just learnt, the answer is based on the arrangement of 14 zeroes and 3 ones (i.e. 17C3)

9) The number of non negative integral solutions of x1+x2+x3 ≤ 10a) 84 b) 286 c) 220 d) none of these

By non-negative integral solutions, the conditions imply that we can have 0 and natural number values for x1, x2, x3, and x4. To remove the sign ≤ add another dummy variable x4. The problem changes tox1+x2+x3+x4=10 This is an example of grouping type1 (Similar to Distinct). It is the arrangement of 10 zeroes and 3 ones. Using the shortcut of zeroes and ones, Therefore the answer is 13C3=286

5) APPLICATION OF FACTORIALSA thorough understanding of Factorials is important because they play a pivotal role not only in understanding concepts in Numbers but also other important topics like Permutation and Combination Definition of Factorial N! = 1x2x3x…(n-1)xn Eg 1) 5!= 1x2x3x4x5=120 eg 2) 3!=1x2x3=6

Let us now look at the application of Factorials

I) Highest power in a factorial or in a product

Questions based on highest power in a factorial are seen year after year in CAT. Questions based on this can becategorized based on the nature of the number (prime or composite) whose highest power we are finding in thefactorial, i.e

a) Highest power of a prime number in a factorial:

To find the highest power of a prime number (x) in a factorial (N!), continuously divide N by x and add all thequotients.

10) The highest power of 5 in 100!

=20; 4; Adding the quotients, its 20+4=24. So highest power of 5 in 100! = 24

ALTERNATIVE METHOD

=20+4=24 (We take upto 52 as it is the highest power of 5 which is less than 100)

b) Highest power of a composite number in factorial

Factorize the number into primes. Find the highest power of all the prime numbers in that factorial using theprevious method. Take the least power.

11) To find the highest power of 10 in 100!Solution: Factorize 10=5*2.1. Highest power of 5 in 100! =24 2. Highest power of 2 in 100! =97Therefore, the answer will be 24, because to get a 10, you need a pair of 2 and 5, and only 24 such pairs areavailable. So take the lesser number i.e. 24 & this is the answer.

12) Highest power of 12 in 100!Solution: 12=22 *3. Find the highest power of 22 and 3 in 100!First find out the highest power of 2.

Listing out the quotients: ; ; ; ; ;

Highest power of 2 = 50 + 25 + 12 + 6 + 3 + 1 = 97. So highest power of 22 = 48 (out of 97 2’s, only 48 are 22)

Now for the highest power of 3. ; ; ; ;

Highest power of 3 = 48. Thus, the highest power of 12 = 48

II) Number of zeros in the end of a factorial or a productFinding the number of zeroes forms the base concept for a number of application questions. In base 10, number of zeros in the end depends on the number of 10s; i.e. effectively, on the number of 5s In base N, number of zeroes in the end is the highest power of N in that product

13) Find the number of zeroes in 13! In base 10Solution: We need to effectively find the highest power of 10 in 13! = Highest power of 5 in 13! As this power

will be lesser.

14) Find the number of zeroes at the end of 15! in base 12.Solution: Highest power of 12 in 15! =highest power of 22 *3 in 15! =Highest power of 3 in 15!= 5

III) Number of factors of any factorialLet us look at an example to understand how to find the number of factors in a factorial

15) Find the factors of 12!STEP 1: Prime factorize 12! i.e. find out the highest power of all prime factors till 12 ( i.e. 2,3,5,7,11).12! = 210*35*52*7*11

STEP2: Use the formula N=am*bn(a, b are the prime factors). Then number of factors= (m+1)(n+1)The number of factors= (10+1)(5+1)(2+1)(1+1)(1+1) =792. Answer=792

APPLICATION QUESTION BASED ON FACTORIAL

16) How many natural numbers are there such that their factorials are ending with 5 zeroes?10! is 1*2*3*4*(5)*6*7*8*9*(2*5). From this we can see that highest power of 5 till 10! is 2. Continuing likethis, 10!-14!, highest power of 5 will be 2. The next 5 will be obtained at 15 = (5*3).Therefore, from 15! To 19! - The highest power of 5 will be 3.20!-24! – Highest Power = 4, In 25, we are getting one extra five, as 25=5*5. Therefore, 25! to 29!, we will gethighest power of 5 as 6. The answer to the question is therefore, 0. There are no natural numbers whosefactorials end with 5 zeroes.

6) USE OF GRAPHICAL DIVISION IN GEOMETRYLet’s look at a technique which will help you solve a geometry question in no time 17) ABCD is a square and E and F are the midpoints of AB and BC respectively.Find the ratio of Area( ABCD): Area(DEF)?

Lets divide the figure using dotted lines as shown in Figure B. Area of ABCD=100%. Area AEGD=50%. Then Area in shaded region 1(AED)= 25%. Similarly, DCFH=50%. Area in shaded region 2(DCF)=25%. Now Area of EOFB= 25%. .Area of shaded region 3(BEF)=12.5%. Total area outside triangle= 62.5%. Area inside triangle= 100-62.5=37.5%. required ratio = 100/37.5 = 8:3. To learn this directly from BYJU, refer the video in the CD given

7) ASSUMPTION METHODThis involves assuming simple values for the variables in the questions, and substituting in answer options based

on those values. Assumption helps to tremendously speedup the process of evaluating the answer as shown

below.

18) k & 2k2 are the two roots of the equation x2 – px + q. Find q + 4q2 + 6pq =a) q2 b) p3 c) 0 d) 2p3

Solution: Assume an equation with roots 1&2 (k=1) =>p (sum of roots)= 3 and q(product of roots)=2. Substitute in q + 4q2 + 6pq = 54. Look in the answer options for 54 on substituting values of p=3 and q=2. we get 2p3 = 54.=>Ans = 2p3.

19) Let ‘x’ be the arithmetic mean and y,z be the two geometric means between any two positive

numbers. The value of is

a) 2 b) 3 c) d)

Assume a GP 1 2 4 8 implies 2 GMs between 1 and 8, i.e. y=2 and z=4.

Arithmetic Mean, .Substitute in . Answer = (23+43)/(2x4x4.5) = 2.

NOTE:- Assume a GP 1 1 1 1 then x=1, y=1 z=1. Answer on substitution=2, which will make the calculation even faster, half of the problems in Algebra can be solved using assumption. This is not direct substitution. In the next eg: see how you can use the same technique in a different question.

20) Consider the set S={2,3,4……2n+1), where n is a positive integer larger than 2007. Define X as the average of odd integers in S and Y as the average of the even integers in S. What is the value of X-Y?

a) 1 b) c) d) 2008 e) 0

The question is independent of n, which is shown below.

Take n=2. Then S= {2,3,4,5). X= 4 and Y=3. X-Y =1, Take n=3. then S={2,3,4,5,6,7}. X=5 and Y=4. X-Y=1 Hence you can directly mark the answer option (a) .You can solve the question in less than 60 seconds.

There were more questions which could be solved using similar strategies. The methods given above clearly show that for someone with good conceptual knowledge and right strategies the quant section is a cakewalk.